The surprising reason our muscles get tired - Christian Moro

譯者: Lilian Chiu 審譯者: SF Huang

You're lifting weights.

你正在舉重。

The first time feels easy,

第一下感覺很輕鬆，

but each lift takes more and more effort until you can’t continue.

但之後每舉一下就會花更多力氣，

直到無法再舉起來。

Inside your arms,

在你的手臂內部，

the muscles responsible for the lifting have become unable to contract.

負責舉重的肌肉無法再收縮。

Why do our muscles get fatigued?

為什麼我們的肌肉會疲勞？

We often blame lactic acid or running out of energy,

我們通常會怪罪於 乳酸或是體能耗盡，

but these factors alone don’t account for muscle fatigue.

但單這些因子並不會造成肌肉疲勞。

There’s another major contributor:

還有另一個重要的原因：

the muscle’s ability to respond to signals from the brain.

肌肉回應大腦訊號的能力。

To understand the roots of muscle fatigue,

要了解肌肉疲勞的根源，

it helps to know how a muscle contracts in response to a signal from a nerve.

最好可以先了解肌肉

如何接收神經的訊號 並做出收縮的回應。

These signals travel from the brain to the muscles in a fraction of a second

這類訊號會在非常短的時間內 從大腦傳送到肌肉，

via long, thin cells called motor neurons.

傳送媒介是又長又細的細胞， 叫做運動神經元。

The motor neuron and the muscle cell are separated by a tiny gap,

運動神經元和肌肉細胞間 有些微的空隙，

and the exchange of particles across this gap enables the contraction.

粒子在空隙間進行交換 就會造成肌肉收縮。

On one side of the gap,

在空隙的一邊，

the motor neuron contains a neurotransmitter called acetylcholine.

運動神經元內有一種 神經傳遞質，叫做乙醯膽素。

On the other side,

另一邊，帶電的粒子或離子，

charged particles, or ions,

line the muscle cell’s membrane:

排在肌肉細胞的細胞膜邊：

potassium on the inside, and sodium on the outside.

內部是鉀，外部是鈉。

In response to a signal from the brain,

收到來自大腦的訊號之後，

the motor neuron releases acetylcholine,

運動神經元的反應是 釋放出乙醯膽素，

which triggers pores on the muscle cell membrane to open.

乙醯膽素會導致 肌肉細胞膜上的孔洞打開。

Sodium flows in, and potassium flows out.

鈉會流進去，鉀會流出來。

The flux of these charged particles is a crucial step for muscle contraction:

這些帶電粒子的流動， 是肌肉收縮的關鍵步驟：

the change in charge creates an electrical signal called an action potential

電荷的改變會創造出電訊號，

叫做「動作電位」， 它會散佈在肌肉細胞中，

that spreads through the muscle cell,

stimulating the release of calcium that’s stored inside it.

刺激儲存在內的鈣釋放。

This flood of calcium causes the muscle to contract

鈣的湧出會造成肌肉收縮，

by enabling proteins buried in the muscle fibers to lock together

因為它會讓肌肉纖維中的 蛋白質鎖在一起，

and ratchet towards each other,

並相互旋扭以拉緊肌肉。

pulling the muscle tight.

The energy used to power the contraction comes from a molecule called ATP.

收縮所需要的能量來自一種 叫做三磷酸腺核苷的分子。

ATP also helps pump the ions back across the membrane afterward,

之後，三磷酸腺核苷也會協助 離子通過細胞膜回到原位，

resetting the balance of sodium and potassium on either side.

讓兩邊的鉀和鈉重新取得平衡。

This whole process repeats every time a muscle contracts.

肌肉每收縮一次， 整個流程就會再重覆一遍。

With each contraction,

每一次收縮，

energy in the form of ATP gets used up,

三磷酸腺核苷形式的能量就會被用光，

waste products like lactic acid are generated,

產生出廢棄產物，如乳酸，

and some ions drift away from the muscle’s cell membrane,

而一些肌肉細胞膜外的離子就會漂離，

leaving a smaller and smaller group behind.

則讓剩下的離子數越來越少。

Though muscle cells use up ATP as they contract repeatedly,

雖然肌肉細胞重覆收縮 會把三磷酸腺核苷用盡，

they are always making more,

但它們持續製造出更多的 三磷酸腺核苷，

so most of the time

所以，通常就算是非常疲勞的肌肉，

even heavily fatigued muscles still have not depleted this energy source.

仍然不會把這個能量用盡。

And though many waste products are acidic,

雖然許多廢棄產物是酸性的，

fatigued muscles still maintain pH within normal limits,

疲勞肌肉的酸鹼值仍然 會維持在正常範圍內，

indicating that the tissue is effectively clearing these wastes.

表示人體組織正有效率地 清除這些廢物。

But eventually, over the course of repeated contractions

但終究在重覆收縮的過程中，

there may not be sufficient concentrations of potassium, sodium or calcium ions

肌肉細胞膜附近可能沒有 足夠濃度的鉀、鈉、鈣離子，

immediately available near the muscle cell membrane

可以及時供給讓系統重置恢復。

to reset the system properly.

So even if the brain sends a signal,

所以，即使大腦送出了訊號，

the muscle cell can’t generate the action potential necessary to contract.

肌肉細胞無法產生出 收縮需要的動作電位。

Even when ions like sodium, potassium or calcium

即使在細胞內或周圍的鈉、

are depleted in or around the muscle cell,

鉀或鈣離子已經耗盡，

these ions are plentiful elsewhere in the body.

身體其他地方仍有相當多這類離子。

With a little time,

they will flow back to the areas where they’re needed,

只要一點時間，它們就會 流回到需要它們的地方，

sometimes with the help of active sodium and potassium pumps.

有時還有活化鈉鉀幫浦 （一種酶）的協助。

So if you pause and rest,

所以，如果你暫停或休息，

muscle fatigue will subside as these ions replenish throughout the muscle.

這些離子補充到肌肉之後， 肌肉疲勞就會消退。

The more regularly you exercise,

你越常規律地運動，

the longer it takes for muscle fatigue to set in each time.

肌肉達到疲勞狀態所需的時間越長。

That’s because the stronger you are,

那是因為當你越強壯，

the fewer times this cycle of nerve signal from the brain

在舉起某特定重量的舉重時，

to contraction in the muscle has to be repeated

大腦重複發送神經訊號讓肌肉收縮的

to lift a certain amount of weight.

次數就會變得越少。

Fewer cycles means slower ion depletion,

循環次數少，就表示 離子耗盡的速度更慢，

so as your physical fitness improves,

所以，當你的體能改善，

you can exercise for longer at the same intensity.

在強度不變的情況下， 你運動的時間就變得更長了。

Many muscles grow with exercise,

許多肌肉會隨著運動而成長，

and larger muscles also have bigger stores of ATP

越大的肌肉也能儲存 越多的三磷酸腺核苷，

and a higher capacity to clear waste,

清理廢物的能力就越強，

pushing fatigue even farther into the future.

感到疲勞所需的時間也就越長。